Video signals are ordinarily recorded on optical discs and magnetic tapes after being FM modulated once.
Such a multivibrator system modulator as shown in FIG. 1 and a frequency converting system modulator as shown in FIG. 2 are well known as FM modulators for use in such a case as described above.
The FM modulator shown in FIG. 1 has a pair of transistors 1 and 2. A video signal as a modulation signal is supplied in common to a base terminal 3 of transistors 1 and 2. Then, an FM modulated output is obtained from an output terminal 4.
The FM modulator shown in FIG. 2 includes an FM modulator 6 having a center frequency of f1 and an FM modulator 7 having a center frequency of f2. The frequencies of the FM modulated outputs of FM modulators 6 and 7 are mixed by a frequency converter 8. Extracted by a low pass filter 9 is a frequency of the difference (f1-f2) of the FM modulated outputs after the frequency mixing. FM modulators 6 and 7 are structured to have frequency deviation directions opposite to each other. That is, FM modulators 6 and 7 are structured such that when one FM modulation frequency is increased with respect to the positive input, the other FM modulation frequency is decreased. Consequently, the frequency of the difference (f1-f2) which is the ultimate FM modulated output has become in practice a frequency of the sum of the center frequencies f1 and f2.
Such FM modulators as shown in FIGS. 1 and 2 are structured to process modulation signals in a purely analog manner, thereby causing the following problems (1)-(4).
(1) A frequency relation between an input voltage and an output voltage is non-linear. PA1 (2) An FM modulation output signal includes a higher distortion (especially secondary distortion). PA1 (3) An oscillating frequency is not stabilized, particularly due to a temperature characteristic. PA1 (4) A modulated signal is leaked to an output side.
Various adjustments and condensors are required to improve the above-described problems, even by which it is difficult to obtain sufficient precision.
Therefore, the object of the present invention is to provide a highly reliable FM modulator of high precision causing none of such problems as described above.